If you like it a bit trippy, this wave pattern might be for you.
Source Ian Soper
Some rectangles, a bit of dust and grunge, plus a hint of concrete.
Source Atle Mo
A playful triangle pattern with different shades of gray.
Source Dimitrie Hoekstra
Design drawn in Paint.net, vectorised using Vector Magic and finished in Inkscape.
Source Firkin
From a drawing in 'In an Enchanted Island', William Mallock, 1892.
Source Firkin
There are many carbon patterns, but this one is tiny.
Source Designova
Tiny little fibers making a soft and sweet look.
Source Badhon Ebrahim
A seamless pattern the unit cell for which can be had by selecting the rectangle in Inkscape and using shift+alt+i.
Source Firkin
ZeroCC tileable stone texture, edited from pixabay, CC0
Source Sojan Janso
Remixed from a drawing in 'Incidents on a Journey through Nubia to Darfoor', F. Ensor, 1891.
Source Firkin
Background Wall, Art Abstract, Block Well & CC0 texture.
Source Ractapopulous
Prismatic Abstract Line Art Pattern Background 2
Source GDJ